How many souls do you think a blessed sibling has?

[fulminating]

I don’t think it is that irrelevant - reattaching bits of the same soul seems to have affected density far more than strength for instance. Formerly ryne until the retcon, the wol, graham and potentially krile are all at 9/14th and not notably stronger than other denizens of the source - or more pertinently the first, ninth and thirteenth.

Taking the two souls thing literally, blessed siblings do seem to stand on a higher baseline than one souled people. Similarly zarol ja, mr snakes from vanguard and the arcadion fighters all seem to get notably stronger wearing a foreign soul. This does seem to also have some pretty bad side effects such as killing you and needing souls to use. I think zarol ja is the only individual we see fight without, with an extra and with a lot, so the main source of extrapolation about how much impact it might have.

There is the potentially similar effect from eating all the light wardens, but I don’t remember that impacting strength, just soul composition until the side effects started and ryne had to use the soul glue.
Likewise it seems that the resonant business is more of a matrix style learnt skill than stat increase, despite using a lot of people as fuel. Since zenos should have had natural echo it does raise a question of why he had the crazy eyes as well, but it might be another side effect. I don’t remember if my cup brimmeth over with ice and fire guy had the same eyes, or the hypertuned but I think they’re a bit different.

[Cleretic]

I don’t think it is that irrelevant - reattaching bits of the same soul seems to have affected density far more than strength for instance. Formerly ryne until the retcon, the wol, graham and potentially krile are all at 9/14th and not notably stronger than other denizens of the source - or more pertinently the first, ninth and thirteenth.

Taking the two souls thing literally, blessed siblings do seem to stand on a higher baseline than one souled people. Similarly zarol ja, mr snakes from vanguard and the arcadion fighters all seem to get notably stronger wearing a foreign soul. This does seem to also have some pretty bad side effects such as killing you and needing souls to use. I think zarol ja is the only individual we see fight without, with an extra and with a lot, so the main source of extrapolation about how much impact it might have.

I might be explaining my read of this badly, but I'll put it this way:

My gut feeling is that your average person in FFXIV's universe might have about the internal capacity to handle five souls' worth of extra aether without having major problems. Five is essentially an arbitrary number here, but it definitely seems to be a relatively low number, going by the void arc in 6.x. The blessing of light (or save that, a warding scale) probably helps in some cases, but they were put into action when dealing with raw aether rather than souls, which might be a different situation entirely. 6.x also suggests some people might just have more capacity for this than others, but it's hard to tell what could be doing this, or if it's even actually a greater capacity and not just, like, a personal indifference to the consequences; the voidsent are weird here. Still, we'll allow that some people might just have a bigger capacity, but I doubt it's by orders of magnitude.

All that means that I ballpark that your average FFXIV-universe person has the capacity to fit a single-digit number of souls in them before things get messy. It seems possible you can notch up the amount there somehow either by personal effort or just pure genetic luck, and there might be differences between shards even outside of the fact the Thirteenth is its own weird thing here; Zoraal Ja's from the Source so that last part doesn't really matter, he's at our baseline anyway.

But it doesn't really matter if your personal capacity is five, eight or ten if you decide to go eat eight hundred of them. I think he gorged himself so enormously beyond capacity that any personal barriers no longer mattered; nobody could possibly be strong enough to eat all those souls and remain okay. It's not a personal failure that he couldn't, although it's probably a personal failure that he tried.